1. Field of the Invention
This invention relates generally to nanocomposites prepared from dispersions of carbon nanotubes (CNTs) in polymeric matrices. It relates particularly to stable dispersions of carbon nanotubes (CNTs) in polymeric matrices employing dispersion interaction, as well as to nanocomposites prepared from these stable dispersions.
2. Description of Related Art
Production of CNT/polymer nanocomposites continues to be hampered by the lack of a reliable method for dispersing the nanotubes in the polymer matrix. As-produced nanotubes exist in bundles of tens or hundreds of individual tubes which are held together by van der Waals interactions. While individual van der Waals interactions are fairly weak, when summed over all of the carbon atoms comprising a nanotube, the cumulative interaction is quite strong, on the order of 1 eV per nanometer of adjacent nanotubes. To achieve the desired properties of CNT/polymer composites, it is important to be able to disperse the tubes individually or in few tube bundles. The problem involves creating stable dispersions in this size regime.
Prior to the innovation described herein, we are aware of no existing method for achieving sufficiently stable dispersions of CNTs for nanocomposite applications without additives such as surfactants or employing chemical modification to the carbon nanotubes. Prior work generally produced metastable dispersions by mechanically mixing nanotubes with polymers using a combination of traditional high shear flow processing techniques and sonication.
A qualitatively different approach to this problem involves the formation of covalent bonds between the matrix polymer and the dispersed CNTs by chemically functionalizing the CNTs. While this approach can indeed produce stable nanocomposites, it has the significant disadvantage of disrupting the extended pi electron system of the CNTs. It is this feature of CNTs which is responsible for their exceptional mechanical and electronic properties. Multifunctionality cannot generally be achieved with chemically functionalized CNTs.
The nanocomposites produced using prior methods are characterized by their metastability. By this we mean that while they might have existed as well dispersed mixtures for some short period of time (days or weeks), the suspended nanotubes eventually reaggregate into larger bundles. The reaggregation process occurs either over time in solution or immediately during common processing operations such as drying or curing. This inevitably leads to reductions in the mechanical and multifunctional properties of the composites.